Modify the river cross section by remeandering the course of the river. Remeandering refers to sections of a river that have been restored to a more natural, meandering course. These reaches often feature sinuous channels with bends and curves, which create a longer flow path for water.
Rivers, lakes and wetlands before NbS have been implemented
Rivers, lakes and wetlands after NbS have been implemented
Remeandering can be applied in channelized rivers, that are also often straightened, widened and incised. These streams are often situated in rural areas.
Remeandering can be used alone or in combination with other NbS for climate change mitigation and adaptation, disaster risk and preparedness, and water management. Often a remeandering is conducted to restore the natural characteristics of the stream channel and may involve that the planform is remeandered and the riverbed level raised that will benefit biodiversity. Remeandered reaches generally have higher water retention capacity due to their meandering nature, complex morphology, and increased interaction with the surrounding area, whereas the water moves faster through channelized reaches with a relative increase in discharge downstream and a higher risk of flooding.
When the river is remeandered, the capacity of the channel to transport water is reduced, leading to a higher water level in the river and more frequent floodings of the surrounding land. With higher water levels in the river, the drainage capacity is reduced, and the groundwater table in surrounding areas rise. This contributes to restore the natural hydrology of the area.
Remeandering can address different types of societal challenges, but the efficiency will depend on the level of the riverbed relative to the surrounding land. If the riverbed continues to be incised relative to the surrounding land, then the benefits will be restricted to the slower water passage through the river (SI).
If the riverbed is raised, then the benefits will be linked to an improved hydrological contact between the river and the surrounding land, and benefits will be linked to both a higher groundwater table in the project area, a slower water passage through the river, and a possibility of flooding of adjacent land within the project area in periods with high water discharge (SII). Below a distinction between SI and SII will be made.
SI: Remeandering can protect downstream areas from flooding . In periods with medium to high levels of precipitation or delay flooding, because the water passage will be slower in the remeandered reach compared to the straightened reach. This benefit can be enhanced if the riverbed is raised (SII).
SII: Remeandering can contribute to a reduction in climate gas emissions if applied in low-lying areas with intermediate to high contents of organic carbon in the soil by reducing CO2 emissions from the area. Furthermore, remeandering can also enhance carbon dioxide sequestration from the atmosphere thereby restoring natural carbon sinks.
Remeandering may also protect downstream areas from flooding in periods with medium to high levels of precipitation, because the slower water passage in the remeandered reach, combined with the higher elevation of the riverbed relative to the terrain improves water retention with the project area thereby protecting flood-prone areas downstream e.g. critical infrastructure, urban or other land uses.
General guidelines in fluvial geomorphology should be followed when remeandering a channelized stream course. The distance between two riffles should be approximately 5-7 times the width of the undisturbed stream channel to promote natural stream dynamics, sediment transport, and habitat diversity. However, this is a general rule, and the specific distance should be planned depending on local conditions such as the width and slope of the river, sediment transport dynamics, and local geomorphological conditions.
Potential outcomes:
SI Protect downstream areas from flooding: Remeandered stream reaches have a longer flow path for water and remeandering can therefore protect downstream areas from flooding in periods with medium to high levels of precipitation. It can also delay flooding by retaining water within the area. This benefit will depend on the length of the remeandered section and the position of the river course relative to terrain. This benefit can be enhanced if the riverbed is raised (see S11 below).
SII Protect downstream areas from flooding: The discharge through the river is reduced, when the surrounding land is flooded. Consequently, water is retained within the project area and the amount of water transported to downstream areas will be reduced. This can be highly beneficial if downstream areas should be protected from flooding like urban or cultivated areas. The efficiency of remeandering for flood protection depends on the length of the river reach that is remeandered, the discharge of the river and the characteristics of the surrounding land, since these parameters will all affect the amount of water that can be retained. The efficiency will be highest in low-laying project areas that are sufficiently large to retain large quantities of water.
SII Reduced climate gas emissions: The reduction in CO2 emissions will be highest in areas where organic soil contents are high (>6%) and where the water level in the river is as close as possible to the surface of the terrain over a large part of the project area. This creates oxygen-free conditions that slow down the decomposition of the organic matter in the soil thereby mediating the largest reduction in CO2 emissions.
SII Reduced nitrogen pollution of aquatic ecosystems: Flooding of areas with nitrate polluted water can stimulate nitrogen removal by denitrification thereby lowering the transport of nitrogen to downstream river reaches, lakes and coastal areas.
Potential side-effects:
SII Methane emission: There is a high risk of methane emissions in areas with standing water. Anaerobic conditions creates favourable conditions for the formation of methane gas through anaerobic decomposition and, as methane is a greenhouse gas, just like CO2, methane emission may counteract the positive effect of less CO2 emission. Therefore, it is very important to maintain a water level just below the surface to minimize this risk.
SII Phosphorus mobilization: When former agricultural land with high contents of phosphorus is flooded there is a high risk of phosphorus mobilization from the soil that can enter the river and cause eutrophication of downstream river reaches, lakes and coastal areas. Therefore, mitigation measures to reduce this risk should be considering before the intervention. This could be harvesting to remove nutrients in the biomass, top soil removal or other measures.
SII Altered hydrology outside the project area: When the groundwater level is raised in a river reach there can be a risk of affecting water level in upstream reaches, drainage pipes and ditches that discharge into the river within the project area. Therefore, the project boundary should be defined so only low-lying areas are included in the project, while higher-lying areas are excluded. This will diminish the risk of negatively affecting drainage conditions outside the project area.
Remeandering is a NbS that can help restore the natural hydrology of an area either alone or in combination with other measures like raising the riverbed level, closure of drainage pipes and ditches. It is therefore an NbS with a high potential for restoring natural characteristics of freshwater ecosystems including many different ecosystem service benefits characterising rewetted areas.
However, to ensure biodiversity net gains within the project area, it is important to be aware that high inputs of nutrients can be critical for many plant species and therefore that biodiversity net gain may not respond positively if this NbS is implemented in combination with closure of drainage pipes and/or ditches at the edge of the project area, thereby increasing the amount of nitrate polluted water entering the root zone of the plants.
Additionally, groundwater dependent vegetation like rich fens may also suffer under prolonged floodings, especially during the growth season, and possible conflicts related to the implementation of other legislation like the habitats directive should therefore be considered.
The costs will vary depending on the local socio-economic-environmental settings including manpower, technology, costs of buying land etc. In addition, there will be operational costs to monitor the effectiveness of the NbS and maintenance costs if adaptive management is needed to maintain the effectiveness over time.
Specific location: Allan Water and peatlands across region
Which ecosystem type(s): Stream and wetland
Title/ name of the Nbs: Floodplain reconnection, Riparian restoration including additions of large woody debris and embankment removal, Channel geomorphology restoration, Rewetting of peatlands, Beaver management, Wetland creation.
Summary: This case study aims to restore a river and the adjacent land and in doing so restore the ecosystems and their functions, while at the same time develop sustainable businesses, tourism and transport. This is achieved by incorporating different nature based solutions including floodplain reconnection, re-meandering, barrier removal and rewetting of peatlands. These nature based solutions will among other result in carbon sequestration and storage and reduced flood risk.
Contact: University of Stirling (emil: forth-era@stir.ac.uk). Further information: https://project-merlin.eu/cs-portal/case-study-17.html
Relevant links to documentation:
https://networknature.eu/casestudy/28918
Forth-ERA at the University of Stirling
Forth Rivers Trust
MERLIN at UKCEH
NatureScot
New LIFE for Welsh Raised Bogs
PeatlandACTION
Kronvang, B., Thodsen, H., Kristensen, E.A., Skriver, J., Wiberg-Larsen, P., Baattrup-Pedersen, A., Pedersen, M.L. and Friberg, N., 2008, June. Ecological effects of re-meandering lowland streams and use of restoration in river basin management plans: experiences from Danish case studies. In Proceedings from the Fourth ECRR conference on River Restoration. Venice–Italy, San Servolo Island (pp. 16-21).
Roley, S. S., J. L. Tank, and M. A. Williams (2012), Hydrologic connectivity increases denitrification in the hyporheic zone and restored floodplains of an agricultural stream, J. Geophys. Res., 117, G00N04. https://doi.org/10.1029/2012JG001950
Pistocchi, A. (ed.), Nature-based solutions for agricultural water management — Characteristics and enabling factors for a broader adoption, Publications Office of the European Union, Luxembourg, 2022, doi:10.2760/343927, JRC131465.
